It's MS Shell Dlg 2 (actually Tahoma on Win XP). Font size: 8pt.
After changing to another font, you can't see this font again (it's not a physical font), so you have to edit the gui.json file in calibre configuration folder. For example, to change from Tahoma to the default font, search for:
Code:
"font": [
"Tahoma",
Replace that with:
Code:
"font": [
"MS Shell Dlg 2",